The aim of this proposal is to understand the structure and connectivity of rat red nucleus. This study will form the basis of a functional study to follow later. Two new findings from this laboratory suggest that the rubro-spinal and rubro-olivary outputs of rat red nucleus are excellent model systems for understanding principles of anatomic organization as well as principles of motor control and compensation. The first finding is that neurons of origin of the rubrospinal tract also project to the inferior olivary nucleus in the medulla (Preliminary Data section and Kennedy 1987a). This was shown using double labelling of the red nucleus neurons. These findings also confirm the previous report from this laboratory of a projection from the red nucleus to the inferior olivary nucleus (paper in Appendix A). The second finding is that the projection from red nucleus to the inferior olivary nucleus is involved in compensation for lesions of the rubrospinal tract. Trained rats could not recover, or only very slowly, after red nucleus lesions destroy the neurons of origin of rubro-olivary and rubro-spinal outputs. In addition, the rubro-olivary output system plays no role in on-going control of movements as tested because, if the rat had already compensated for a rubrospinal tract lesion, its performance was minimally and only temporarily affected after a subsequent red nucleus lesion. The complete anatomical description of this system as proposed here using multiple tracers will provide a guide to functional studies and their subsequent interpretation. Ultimately, it is expected that understanding the fundamental nature of the red nucleus and, in particular, its role in compensation for neurological deficits, will lead to a rational basis for treatment modalities for such disorders as stroke and learning disabilities.